The present invention relates to the field of communications, which is slow-growing, very competitive and essentially mature. For purposes of this specification, communications shall refer to multi-media transmission of information from one or more nodes to one or more other nodes. In particular, this invention relates to a system for transmitting voice, image, text and data from one computer to another, where either or both of the computers can be or include an ordinary household telephone.
A. Switching
At the heart of any modern communications system is switching. The architecture of the first automatic switching system has some basic characteristics, having long range implications. The first step-by-step switching system (i.e., rotary switching system) actually establishes the desired connection between nodes by remote control from a telephone dial. Such a system is said a self-connecting, distributed control system. In the beginning, it was electromechanical by nature and therefore difficult to maintain. In addition, the remote control of relays employed by the system generated impulse noise at the telephone exchanges, which became the main source of noise which in turn caused errors in received computer data when transmitted over telephone lines.
In a crossbar switching system, the rotary switches of the step-by-step switching system were replaced with matrix switches, which were easier to maintain and produced less noise (Electromagnetic Interference or EMI) when used for data transmission. The address of the called party, generated by the telephone dial, was stored in a register and processed through relay logic for making connection via the matrix switch. Later, the crossbar switch was replaced by Reed relays. Later still, the relay logic was implemented electronically, which resulted in the electronic switching system (ESS). ESS is fast, noise-free and easy to maintain.
Step-by-step, crossbar and ESS switching systems are known as "space division switching". Time division switching systems are an outgrowth of digital transmission technologies. Such switching systems allocate time slots to users for the duration of a connection. All users are physically connected to the same communication line, but have time slots allocated for the duration of the call. Such a switching system is economically attractive and, partly because it uses substantially less wiring, is easy to maintain. However, the bandwidth of time division switching systems must be divided among all the simultaneous users connected to the line. Thus, in spite of high-speed technology available today, the bandwidth or, alternatively, digital speed for each user, is limited to 64K bits/sec.
Time division switching is a natural outgrowth in the evolution of switching systems. For telephony, it provides quality and security at reasonable cost. User-oriented functions can be provided with the elegance and flexibility of computer-controlled switching systems. For computer users, data can be transmitted at 64K bits/sec on a switched bases, and facsimile systems can be faster and more powerful.
With the advent of digital trunk carrier systems and digital switching, it quickly became clear that communication networks would evolve toward a capability to provide end-to-end digital connections. Much effort has been expended, worldwide, to define a set of realizable standards for what is called an Integrated Services Digital Network (ISDN). The ISDN concept permits endusers to transmit up to a total of 144K bits/sec of information consisting of two 64K bit/sec channels, which can support circuit or packet switching, and a third 16K bit/sec packet-switched channel which makes use of existing two wire loop systems in most cases. The 16K bit/sec packet switching channel has a well defined protocol and is used for both signalling between endusers and the central office switch, and for user-to-user packet information.
ISDN will provide digital voice and data services far superior to anything available today. From a computer communications point of view, ISDN is very attractive, since it provides 64K bits/sec switched service without a modem. While facsimile services are greatly improved, they can, at best, provide only 16K bits/sec over analog voice networks on a worldwide basis. Furthermore, when the potential capability of optical fibers is considered, together with its ultimate availability to every telephone user at a cost roughly equivalent to copper transmission lines, the capabilities of the ISDN switching pales in comparison with the gigabit transmission capabilities of fiber optic transmission.
Therefore, a high speed digital switching system capable of providing fast access to data for high and low speed computer terminals, access to image files and facilitating communications for all kinds of compatible and incompatible computer systems at a cost of switching affordable for digital voice communication is desirable. Even more desirable is such a system which is compatible with and transparent to ISDN facilities and end users, but which anticipates conversion of national and worldwide telecommunications networks from two wire, copper linkages to optical fibers and ultra-fast switching systems.
An historical review of the prior art of telecommunications in the U.S. is given in "Communications and Switching" by Stewart D. Personick and William 0. Flechenstein, Proceedings of the IEEE, Vol. 75, No. 10, October, 1987. In addition, ISDN is more fully described and discussed in IEEE Communications Magazine, Vol. 25. No. 12, December, 1987.
B. Telephony
In the 1930's, basic standards for toll quality telephony were established. That basic standard comprised the minimum bandwidth needed to assure recognition of the speaker by the receiver at the other end of the link, together with at least 98% understandability of the speech in context. The minimum bandwidth was 300 Hz to 3400 Hz, which resulted in 4 kHz frequency spacing for single sideband (SSB) cable and radio transmission. These standards have been preserved in digital transmission, using pulse code modulation (PCM), and are perpetuated in ISDN standards.
Toll quality telephone sounds astonishingly good in spite of the relatively narrow (approximately 3 kHz) bandwidth, where modern transducer technologies, such as the electret microphone and dynamic earphone, are used in the user's handset Such a telephone link transmits all the vowels very well. However, transmission of consonants, which have main speech energies concentrated between 7 kHz to 8 kHz, is rudimentary at best. Generally, speech taken in context provides sufficient clues for good understandability, although, unexpected words and names typically must be spelled in order to circumvent the lack of bandwidth in toll quality telephone connections. Thus, in general, telephone networks having a high-fidelity link at a cost equal to or less than the user pays today is, at least, desirable.
According to information theory when PCM was discovered, the sampling rate of an analog signal was set at 2W for perfect recovery of signals having a bandwidth of less than W. In order to prevent foldover intermodulation distortion, the speech spectrum had to be strictly limited to less than 4 kHz. Thus, the sampling rate for voice telecommunications was set at 8K samples/sec, and a prior art encoder, utilizing an advanced Adaptive Differential PCM (ADPCM) module for digitizing analog voice signals at that rate, is shown in FIG. 1.
In order to strictly limit the speech spectrum to 4 kHz, a sharp, low pass filter was required as also shown in FIG. 1. In addition, digital encoding of speech was very costly and could be economically justified only for Time Division Multiplexing (TDM) transmission systems. While single chip encoders are now available on the market which make digital telephones economically feasible, the sharp low pass filter required for classical PCM encoders requires about half the semiconductor "real estate" of a typical coder/decoder (codec) chip.
C. Telephone Graphics
The inability to draw simple pictures remotely is a severe limitation of present-day telephony. Even with a hi-fi telephone, the ability to communicate is still hampered by the absence of graphics capability. The first serious attempt to provide remote telephone graphics was the "Picture Phone", introduced by Bell Laboratories. While the Picture Phone was a technical success, the failure in the marketplace is easily attributed to its cost and inability to satisfy a well-defined need. More simply stated, the market requirements were not properly defined before the Picture Phone was developed. However, even today, there are other similar attempts at transmitting video over presently installed telephone lines. See, for example, the VisiTec Visual Telephone Display, manufactured by Mitsubishi.
The ability to remotely present graphics, including charts, in real time while the telephone conversation is in progress and at a reasonable cost is extremely desirable. Definition of telephone graphics is virtually at the same level of development that definition of basic standards for toll quality telephony was in the 1930's. Once defined, this new video service can be expanded to higher resolution, including gray scale, color and motion as required. Thus, development of a standard for telephone graphics, preferably based on presently available technology but which anticipates technology advances, is desirable.
D. System Considerations
The computer user communications traffic may appear to be profoundly different from telephone voice traffic requirements. For example, the computer terminal user typically establishes connection to a computer port at the beginning of the day and maintains the connection for some hours until he goes home. Therefore, 100% connectivity or usage of the switching systems is indicated. On the other hand, telephone user statistics indicate that the average telephone call is about 10 minutes long, and that only about 10% of all users need simultaneous connections at the same time.
A closer look at the actual information traffic indicates that data is transmitted between the terminal and the computer system occur less than 10% of the time the linkage is established. However, when the data is generated, it should be transmitted very quickly. The desired data transmission rates are in the megabits/sec range and the desired connection times are less than a microsecond. Thus, the use of a telecommunications system by a computer terminal user is actually much more like the use of the same system by a telephone user than has been appreciated by telecommunications systems designers in the past.
Even if a switch controller, designed and constructed in accordance with ISDN specifications, is built in high speed technology, and interfaces were designed to accommodate both the computer terminal user (text and data) and the telephone (voice and image information), at least 10,000 programming steps are required before a single connection is actually made. Moreover, even if the system operated at 10,000,000 instructions/sec (10 MIPS), the connect time still will be in the 1 millisecond range. While millisecond connect times are much faster than any presently known voice switching system, it is inadequate to accommodate the nanosecond switching requirements of high speed systems which will be available in the foreseeable future, perhaps in accordance with the present invention.